1. Field of the Invention
The present invention relates to pharmaceutical compositions containing nitroxide compounds useful in ameliorating the deleterious effects of toxic oxygen-related species in living organisms, and methods of using the same.
2. Description of Related Art
The utilization of oxygen by mammals carries both a blessing and a potential curse. The blessing is that all mammals require oxygen for life. The potential curse is that during the metabolism of oxygen, a variety of toxic oxygen-related species such as hydroxyl radical, (.OH); hydrogen peroxide, (H.sub.2 O.sub.2); and superoxide, (O.sub.2) are produced. Left unchecked, these free radical species could undoubtedly damage cells. However, cells have evolved elaborate detoxification and repair systems to rid themselves of these potentially toxic and undesirable metabolic by-products: superoxide dismutase (SOD) can convert superoxide to H.sub.2 O.sub.2, and catalase (CAT) can convert H.sub.2 O.sub.2 to H.sub.2 O.
Yet another means to detoxify H.sub.2 O.sub.2 (and organoperoxides) is via the enzyme glutathione peroxidase (GPX), which with glutathione (GSH), also converts H.sub.2 O.sub.2 to H.sub.2 O. Glutathione transferase (GST), in addition to its ability to conjugate and inactivate drugs and xenobiotics, also possesses peroxidase activity and can detoxify H.sub.2 O.sub.2. These systems represent the major detoxification pathways for oxygen-derived free radicals species; however, there are doubtless other systems that may provide protection including protein sulfhydryls and other thiol-related enzymes that could be involved in repair mechanisms.
Despite the efficiency of these enzymatic systems, there is a small "leakage" of toxic species beyond the biochemical defense network. Of particular importance is the ultimate fate of H.sub.2 O.sub.2 should it escape detoxification. H.sub.2 O.sub.2, itself an oxidant capable of damaging biologically important molecules, can also undergo reduction via ferrous complexes to produce .OH. This reaction (often referred to as Fenton chemistry) produces the highly reactive .OH, which in the order of 10.sup.-9 seconds, can: 1) abstract electrons from organic molecules; 2) break chemical bonds; 3) initiate lipid peroxidation; and 4) react with another .OH to produce H.sub.2 O.sub.2. It is not known whether chronic exposure to low level oxygen-derived free radical species is deleterious; however, it is postulated that the process of aging may be a manifestation of the organisms's inability to cope with sustained oxidative stress. Many modalities used in cancer treatment including x-rays and some chemotherapeutic drugs exert their cytotoxicity via production of oxygen-related free radicals, thereby imposing an added burden to normal detoxification systems. Additionally, free radicals and toxic oxygen-related species have been implicated in ischemia/reperfusion injury, and have long been thought to be important in neutrophil-mediated toxicity of foreign pathogens. Likewise, free radical damage has been implicated in carcinogenesis. The term "oxidative stress" has thus emerged to encompass a broad variety of stresses, some of which have obvious implications for health care.
There has been considerable interest in devising additional approaches, apart from inherent intracellular detoxification systems, to protect cells, tissues, animals, and humans from the toxic effects of any agent or process that imposes oxidative stress. In the past few years, experimental studies have indicated that enzymes such as catalase and superoxide dismutase, and agents such as allopurinol and metal chelating compounds, afford protection against oxidative stress. None of these approaches is at present being applied to humans.
The application of the nitroxides of the instant invention is novel in this respect, and affords several unique advantages. Although the group of chemical compounds called stable nitroxide spin labels has had extensive biophysical use, they have never been used as antioxidants. They exhibit low reactivity with oxygen itself. Being low molecular weight, uncharged, and soluble in aqueous solution, they readily cross into the intracellular milieu. Enzymes such as catalase and superoxide dismutase do not. Therefore, the nitroxides should be superior to catalase and superoxide dismutase in that they can exert protection inside the cell. They are active within the biological pH range of about 5 to 8. Nitroxides are not proteins; therefore, the possibility of antigenic stimulation is remote. Previous low molecular weight superoxide dismutase mimics have all been metal dependent. The current agents do not contain metals, and problems with dissociation constants and deleterious metal induced reactions are therefore avoided. These compounds are apparently non-toxic at effective concentrations, and their lipophilicity can be controlled by the addition of various organic substituents, facilitating targeting of the molecules to specific organs or organelles where toxic oxygen-derived species are generated, to regions which are particularly susceptible to oxidative damage, or to the brain, if this is so desired. Previous radiation protectors have been sulfhydryl-group dependent. The current agents do not have a sulfhydryl group. Finally, previous use of these types of compounds as magnetic resonance contrast agents does not relate to their instant application as antioxidants.